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Forbes : We Probably Wouldn't Even See A Doomsday Asteroid Until It Was Too Late [...] So how much warning will we really have if an asteroid is about to hit Earth? The answer is pretty grim.

With so many of even the larger Near Earth Objects remaining undiscovered, the most likely warning today would be zero.

Is this an accurate description? Are we unlikely to see an asteroid that does serious damage to our civilisation beforehand if it were to strike earth?

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"Too Late" Too late for what? To late to send Bruce Willis to drill a bomb into it? ;) –  Suma Jan 11 '13 at 10:49
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Irony aside, I think the clarification needed is how much time you would assume "soon enough". Reading the article, perhaps a the quote could extended to contain the context which clarifies it: ”We would see nothing at all until suddenly, just as the impact occurred, we noticed a flash of light and the shaking of the ground as it hit." Still, some quantification seems necessary. What kind of "unlikely" are we talking about? An asteroid hit by itself is already quite unlikely. –  Suma Jan 11 '13 at 11:12
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@Suma : I don't think this question would get improved by getting more specific and thereby possible reducting the amount of interesting data that can be used to answer it. It's specific enough to be clearly answerable (if anything quantifying serious damage would make more difference then quantifying the timeframe) –  Christian Jan 11 '13 at 13:54
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This depends upon how much space you are looking at and how often you examine the same part of the sky. This is a question that could lead to some very interesting answers. –  rob Jan 11 '13 at 14:02
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Note that depending on your ability to shift the things, "too late" could be years of advanced warning... –  dmckee Jan 11 '13 at 16:50
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1 Answer

Don't Panic

The article in question has the correct answer:

Any NEO that is going to hit the Earth will swing near our planet many times before it hits, and it should be discovered by comprehensive sky searches like Spaceguard.

There are many all-sky survey projects that would likely detect any civilization-damaging asteroid. And if the survey projects detect such an asteroid, it is almost certainly the case that they would discover it many years prior to impact. More all-sky surveys with better and better detection techniques will be coming online in the next decade and increase our chances of spotting a potential impactor. As surveys increase, we can anticipate a rise in reports of "near misses" and "potential impact" stories, even though what will really happen is that our chance of being caught unawares is reduced.

So to put some numbers and references to the previous paragraph:

The Torino Scale describes asteroid impact hazards. To achieve "serious damage to our civilization," as specified in the question, would require a Torino Scale 9 or 10 impact. Such impacts occur on average every 10,000-100,000 years.

We already have all-sky surveys that are specifically trying to trace "Near Earth Objects" and potential impactors. These are finding NEOs at an increasing rate (and this is likely to continue), but most large NEOs have probably been found, as shown in this image:

large NEO discovery rate total NEO discoveries

We're discovering fewer and fewer large NEOs even as we're discovering more and more total NEOs. This indicates that the chances of us having missed a "sneak" Torino Scale 9 or 10 asteroid are increasingly slim.

In combination with being an undiscovered large NEO, the doomsday asteroid would additionally need to be in an orbit that hits us quickly. That's highly unlikely: of the almost 10,000 NEOs detected to date, none has ever impacted. None is even anywhere near likely to impact.

The probability of a doomsday asteroid sneaking up on us is the product of small probabilities: the odds of us having missed a large NEO * the odds of it having a bulls-eye course.

In summary, although there are many, many asteroids out there, the vast majority of which are undiscovered, we benefit from an important fact:

Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

-- Douglas Adams

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How common or uncommon are large objects entering the (inner) solar system at a high speed from very far away (e.g. interstellar space)? If such exist, we might see them only much shorter in advance? –  gerrit Jan 11 '13 at 21:17
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The frequency of cold rocks in interstellar space is highly speculative since they're dark and cold and far away (they recently discovered that the number of brown dwarfs is much bigger than expected and they're huge relative to asteroids). I think it's logical that large numbers exist (albeit in a huge volume) and that they occasionally traverse our SS or get captured but it must be rare. AFAIK there's no evidence that any meteorite is of extrasolar origin and I'm fairly positive that there's never been an asteroid or comet suspected of such. –  Larry OBrien Jan 11 '13 at 22:22
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